A tunnel FET (tFET) is a transistor obtaining an on-current attributed to an interband tunnel current. A swing and a standby current of the tunnel FET are small compared with a conventional FET which obtains an on-current attributed to a drift current or a diffusion current in an inversion region.
However, the tunnel FET has a problem that the on-current of the tunnel FET is small compared with the conventional FET. In a case where a circuit is configured with FETs whose on-currents are small such as tunnel FETs, the operation of the circuit becomes slow. Therefore, various methods have been proposed to increase the on-current of the tunnel FET.
For example, there has been proposed a method in which an n type shallow region is formed on a p type source region in a semiconductor substrate to increase an area in which the tunnel current flows. In this method, however, a sufficient amount of on-current is not available unless a substrate area occupied by the tunnel FET is increased. In addition, the tunnel current flows through a side surface of the n type shallow region as well as a lower surface of the n type shallow region. This increases an off-current of the tunnel FET.
There has been proposed another method in which a source region is formed of a material having a narrow bandgap to make the tunnel current easy to flow. However, this method makes the off-current easy to flow as well as making the on-current easy to flow.